The present invention generally relates to servo type accelerometers for detecting acceleration, and in particular to a servo type accelerometer in which a pendulum is servo-controlled by a torque generating unit in which a current corresponding to acceleration is supplied to a coil which is a part of the torque generating unit.
Currently, accelerometers for detecting the presence or quantity of acceleration are widely used in various fields such as automobiles and navigation systems.
For example, the Japanese Laid-Open Patent Application No. 90111/1983 discloses a servo type accelerometer. This type of accelerometers detects a rotational displacement of a pendulum proportional to the acceleration, and provides a current corresponding to the detected displacement with a coil which constitutes the torque generating unit together with a pole piece (yoke). A torque produced by the coil is exerted on the pendulum to drive it toward its original position where there is no accelerationl. The amount of the current flowing in the coil corresponds to the acceleration. The displacement of the pendulum is detected by a displacement detecting unit including one light-emitting element and two mutually independent photoelectric elements. A light interrupting member is provided between the light-emitting element and the photoelectric elements. The light interrupting member of the pendulum deviates from its original position as a function of the acceleration. Therefore, a difference between outputs of the two photoelectric elements corresponds to the acceleration. It should be noted that there is the requirement of employing two photoelectric elements having almost the same characteristics relating to the light sensitivity, leakage current, temperature and the like.
However, the above accelerometer has the following disadvantages. The accelerometer employs two independent photoelectric elements. That is, each of the two photoelectric elements has a photoelectric part which is packaged. In general, the possibility that two photoelectric elements which are arbitrarily selected from among a number of elements have almost the same characteristics is very rare. In most cases, two arbitrarily selected photoelectric elements are considerably different from each other in characteristics such as photo sensitivity, temperature, leakage current and so on. Especially, the temperature characteristics are extremely different, depending on the photoelectric elements. Therefore, various adjustments on the displacement detecting unit are required at the time of assembly. In addition, the acceleration detecting properties of the displacement detecting unit is degraded as time elapses.
In addition, the torque generating unit of the conventional servo type accelerometer uses the pole piece (yoke) of a cylindrical shape. Therefore, it is very difficult to constitute the servo type accelerometer of a compact size, especially, a thin accelerometer.
Moreover, the conventional accelerometer employs two sets of torque generating units to balance the pendulum. For this reason, the assembly and adjustment operation are complicated.